The present invention relates to a jet pump, and more particularly, to a jet pump for use with an in-tank fuel pump system for an automobile.
In recent years, an increasing number of automobiles have included fuel systems wherein the fuel pump for the system is incorporated within the fuel tank of the automobile. In such systems, the fuel pump is typically located within a canister in the fuel tank and the canister is overfilled with fuel supplied from a fuel return line for returning an oversupply of fuel from the automobile's engine. As the fuel returns from the engine through the return line, it is typically caused to pass through a venturi orifice and into an inlet passage leading into the canister. The inlet passage is submerged in fuel within the fuel tank and the fuel exiting the venturi orifice creates a pressure drop in the area of the inlet passage such that additional fuel from the fuel tank is conveyed into the canister along with fuel jetted from the venturi orifice into the inlet passage. The amount of fuel returning through the fuel return line generally varies from between 20 to 180 liters/hr. Currently, venturis are of a low performance design wherein the average aspirated fuel is sufficient to keep the fuel canister filled under most operating conditions. However, there is a trend toward providing higher performance venturis which maintain sufficient canister filling at fuel return line flow rates as low as 20 liters/hr.
The venturis currently in use are generally formed as fixed nozzles wherein the orifice size is optimized for the anticipated use. In order for the venturi to draw sufficient fuel into the canister from the fuel tank along with the fuel stream from the venturi, it is necessary to select a small enough orifice size for the venturi to provide the required pressure drop across the orifice to draw fuel from the fuel tank into the inlet passage of the canister. Thus, in order to increase the flow rate of fuel into the canister at low fuel return line pressures, it is necessary to decrease the orifice size of the venturi whereby a larger pressure drop across the venturi orifice is obtained and a larger flow of fuel from the fuel tank is drawn into the canister inlet passage.
A short coming of the present jet pump designs is that as the venturi orifice size is decreased, the back pressure within the fuel return line is increased. Accordingly, when the venturi orifice size is optimized for low fuel return line flow rates, there is a danger of an excessive back pressure being produced when the flow rate through the fuel return line increases to its maximum value. This is particularly critical in applications wherein the fuel return line is formed of plastic and may crack or rupture as a result of excessive pressure.
An additional desired characteristic of in-tank fuel pump systems is the provision of a check valve or other means for preventing flow in a reverse direction from the tank through the fuel return line. Such a check valve should be preferably incorporated into the venturi structure to minimize complexity of the fuel system.